Optical coherence tomographic device and light source

ABSTRACT

An optical coherence tomographic device configured to acquire a tomographic image of a subjected eye is disclosed herein. The optical coherence tomographic device may include a light source of wavelength sweeping type; and a measurement optical system configured to irradiate the subjected eye with light outputted from the light source, where D/S×λ&gt;1.61 may be satisfied, with a diameter of the light outputted from the light source at an incident position to the subjected eye is D, a wavelength swept frequency of the light source is S, and a center wavelength of the light outputted from the light source is λ.

CROSS-REFERENCE

This application claims priority to Japanese Patent Application No.2017-246663, filed on Dec. 22, 2017, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The technique disclosed herein relates to an optical coherencetomographic device and a light source used for the same.

BACKGROUND ART

Optical coherence tomographic devices configured to measure an axiallength of a subjected eye and positions of parts in the subjected eyehave been developed. For example, an optical coherence tomographicdevice of Japanese Patent Application Publication No. 2017-176842 isprovided with a measurement optical system configured to irradiate asubjected eye with light outputted from a light source and guidereflected light from the subjected eye, and a reference optical systemconfigured to irradiate a reference surface with the light outputtedfrom the light source and guide reflected light from the referencesurface. In measurement, a position of a target part in the subjectedeye is specified from interference light in which the reflected lightguided by the measurement optical system is combined with the reflectedlight guided by the reference optical system.

SUMMARY

In an optical coherence tomographic device, such as the opticalcoherence tomographic device of Japanese Patent Application PublicationNo. 2017-176842, a subjected eye is normally irradiated with lighthaving a relatively small diameter of 1 to 2 mm. However, if acrystalline lens of the subjected eye has an opacified portion, forexample, due to cataract, the light outputted from a light source to thesubjected eye may attenuate at the opacified portion, and an amount oflight reaching a retina may become small. As a result, it has beenproblematic that detection of reflected light from the retina isdifficult and thus measurement for a position of the retina, an axiallength and the like is difficult. In view of this, the disclosure hereindiscloses a technique capable of increasing a signal strength ofreflected light from a retina of a subjected eye.

An optical coherence tomographic device disclosed herein may beconfigured to acquire a tomographic image of a subjected eye. Theoptical coherence tomographic device may comprise a light source ofwavelength sweeping type; and a measurement optical system configured toirradiate the subjected eye with light outputted from the light source.D/S×λ>1.61 may be satisfied, with a diameter of the light outputted fromthe light source at an incident position to the subjected eye is D, awavelength swept frequency of the light source is S, and a centerwavelength of the light outputted from the light source is λ.

In the above optical coherence tomographic device, signal receptionsensitivity can be increased by irradiating the subjected eye with lightthat satisfies the above-mentioned condition. Due to this, a signal ofreflected light from a retina can be obtained with high sensitivity.

A light source disclosed herein may be of wavelength sweeping type, maybe equipped in an optical coherence tomographic device configured toacquire a tomographic image of a subjected eye and may be configured tooutput light with which the subjected eye is irradiated. With a diameterof light outputted from the light source at an incident position to thesubjected eye is D, a wavelength swept frequency is S, and a centerwavelength is λ, the light source may be configured to be capable ofadjusting at least one of the wavelength swept frequency S and thecenter wavelength λ so that D/S×λ>1.61 is satisfied.

The above light source can adjust at least one of the wavelength sweptfrequency S and the center wavelength λ so as to irradiate the subjectedeye with light that satisfies the above-mentioned condition. Due tothis, the light source can bring similar operation and effect to thoseof the above optical coherence tomographic device.

Another optical coherence tomographic device disclosed herein may beconfigured to acquire a tomographic image of a subjected eye. Theoptical coherence tomographic device may comprise a light source ofwavelength sweeping type; and a measurement optical system configured toirradiate the subjected eye with light outputted from the light source.A diameter D of the light outputted from the light source at an incidentposition to the subjected eye may be 3 mm or more.

In the other optical coherence tomographic device above, an amount oflight reaching a retina can be increased since the diameter D of thelight outputted from the light source at the incident position to thesubjected eye is 3 mm or more. Due to this, even when a crystalline lensof the subjected eye has an opacified portion, a signal of reflectedlight from the retina can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a schematic configuration of an opticalsystem of an optical coherence tomographic device of an embodiment.

FIG. 2 is a block diagram showing a control system of the opticalcoherence tomographic device of the embodiment.

FIG. 3 is a diagram for explaining a function of a 0-point adjustmentmechanism.

FIGS. 4A, 4B, and 4C show diagrams for explaining a function to change adiameter of incident light to a subjected eye by a focal pointadjustment mechanism.

FIGS. 5A and 5B show diagrams schematically showing incident light to asubjected eye with its crystalline lens having an opacified portion with5A showing a case with a small diameter of light and 5B showing a casewith a large diameter of light.

FIG. 6 shows diagrams for explaining a procedure of processing aninterference signal waveform.

DETAILED DESCRIPTION

Representative, non-limiting examples of the present invention will nowbe described in further detail with reference to the attached drawings.This detailed description is merely intended to teach a person of skillin the art further details for practicing preferred aspects of thepresent teachings and is not intended to limit the scope of theinvention. Furthermore, each of the additional features and teachingsdisclosed below may be utilized separately or in conjunction with otherfeatures and teachings to provide improved optical coherence tomographicdevices and light sources, as well as methods for using andmanufacturing the same.

Moreover, combinations of features and steps disclosed in the followingdetailed description may not be necessary to practice the invention inthe broadest sense, and are instead taught merely to particularlydescribe representative examples of the invention. Furthermore, variousfeatures of the above-described and below-described representativeexamples, as well as the various independent and dependent claims, maybe combined in ways that are not specifically and explicitly enumeratedin order to provide additional useful embodiments of the presentteachings.

All features disclosed in the description and/or the claims are intendedto be disclosed separately and independently from each other for thepurpose of original written disclosure, as well as for the purpose ofrestricting the claimed subject matter, independent of the compositionsof the features in the embodiments and/or the claims. In addition, allvalue ranges or indications of groups of entities are intended todisclose every possible intermediate value or intermediate entity forthe purpose of original written disclosure, as well as for the purposeof restricting the claimed subject matter.

Some of the features characteristic to below-described embodiments willherein be listed. It should be noted that the respective technicalelements are independent of one another, and are useful solely or incombinations. The combinations thereof are not limited to thosedescribed in the claims as originally filed.

(Feature 1) In an optical coherence tomographic device disclosed herein,a diameter D may be 3 mm or more.

(Feature 2) In the optical coherence tomographic device disclosedherein, a wavelength swept frequency S may be 10 Hz or more and 5000 Hzor less. According to such a configuration, signal reception sensitivitycan be further improved, and occurrence of artifacts can be suppressed.

(Feature 3) In the optical coherence tomographic device disclosedherein, a center wavelength λ may be 700 nm or more and 1400 nm or less.According to such a configuration, intensity of light can be suppressedfrom attenuating while the light propagates within an eye, and thesignal reception sensitivity can be further improved.

(Feature 4) The optical coherence tomographic device disclosed hereinmay further comprise a controller configured to control the lightsource. The light source may be configured to be capable of adjusting atleast one of the wavelength swept frequency S and the center wavelengthλ. The controller may be configured to adjust at least one of thewavelength swept frequency S and the center wavelength λ of the lightoutputted from the light source. According to such a configuration,D/S×λ>1.61 can be easily satisfied by adjusting at least one of thewavelength swept frequency S and the center wavelength λ of the lightoutputted from the light source according to the diameter D of the lightoutputted from the light source at an incident position to a subjectedeye.

(Feature 5) In the optical coherence tomographic device disclosedherein, a measurement optical system may be configured to be capable ofadjusting the diameter D of the light outputted from the light source atthe incident position to the subjected eye. The measurement opticalsystem may be configured to adjust the diameter D so that the diameter Dbecomes equal to or less than a pupil diameter of the subjected eye.According to such a configuration, the diameter D of the light at theincident position to the subjected eye can be adjusted according to thepupil diameter of the subjected eye. Due to this, the subjected eye canbe irradiated efficiently with the light.

(Feature 6) In the optical coherence tomographic device disclosedherein, the measurement optical system may comprise a focal pointadjuster configured to be capable of adjusting a focal position of thelight outputted from the light source. According to such aconfiguration, the diameter D can be easily adjusted by using the focalpoint adjuster.

The optical coherence tomographic device disclosed herein may furthercomprise a light receiving element configured to receive reflected lightfrom the subjected eye and to output a signal which corresponds to anintensity of the reflected light; a sampling circuit configured tosample the signal outputted from the light receiving element; and asample clock generator configured to generate a clock signal whichdefines a timing for sampling the signal based on a frequency of thelight outputted from the light source. According to such aconfiguration, distortion in the sampled signal can be suppressed, and atomographic image with higher resolution can be obtained.

Embodiment

An optical coherence tomographic device of an embodiment will bedescribed hereinbelow. As shown in FIG. 1, the optical coherencetomographic device comprises a light source 12, a measurement unit 10configured to examine a subjected eye 100, and a K-clock generator 60.Light outputted from the light source 12 enters a beam splitter 18 andis split into light to be guided to the measurement unit 10 and light tobe guided to the K-clock generator 60 in the beam splitter 18.

The measurement unit 10 comprises an interference optical system 14configured to cause reference light interfere with reflected light thatis reflected from the subjected eye 100, an observation optical system50 configured to observe an anterior part of the eye 100, and analignment optical system (not shown) configured to align the measurementunit 10 with respect to the subjected eye 100 in a predeterminedpositional relationship. An alignment optical system that has been usedin a well-known optical coherence tomographic device can be used as theaforementioned alignment optical system, and thus detailed explanationthereof is herein omitted.

The interference optical system 14 is constituted of a measurementoptical system configured to irradiate the subjected eye 100 with lightfrom the light source 12 and guide reflected light therefrom, areference optical system configured to irradiate a reference surfacewith light from the light source 12 and guide reflected light therefrom,and a light receiving element 26 configured to receive interferencelight in which the reflected light guided by the measurement opticalsystem is combined with the reflected light guided by the referenceoptical system.

The light source 12 is of wavelength sweeping type, and a wavelength oflight outputted therefrom changes with a predetermined period. When thewavelength of the light outputted from the light source 12 changes, areflection position of reflected light, among reflected lights fromrespective parts of the subjected eye 100 in its depth direction, thatcauses interference with the reference light changes correspondingly tothe wavelength of the outputted light. This change in the reflectionposition takes place in the depth direction of the subjected eye 100.Therefore, positions of the respective parts (that is, a crystallinelens 104, a retina 106 and the like) inside the subjected eye 100 can bespecified by measuring the interference light while changing thewavelength of the outputted light.

The light source 12 may be configured to output light having awavelength (specifically, a center wavelength) of 700 nm or more and1400 nm or less. Light having a wavelength of less than 700 nm isvisible. Therefore, by outputting the light having the wavelength of 700nm or more, a subject does not have to be dazzled and a pupil of thesubjected eye 100 can be suppressed from constricting. Further, lighthaving a wavelength of more than 1400 nm is likely to be absorbed bywater. Therefore, by outputting the light having the wavelength of 1400nm or less, intensity of the light can be suppressed from attenuatingwhile the light propagates within the eye, and detection sensitivity forlight received by the light receiving element 26 can be suppressed fromdecreasing. In the present embodiment, the light source 12 outputs lighthaving a wavelength of 700 nm. Further, a wavelength swept frequency ofthe light source 12 may be 10 Hz or more and 5000 Hz or less. With a lowwavelength swept frequency, the detection sensitivity for light receivedby the light receiving element 26 is improved. Therefore, by setting thewavelength swept frequency to 5000 Hz or less, the light receivingelement 26 can detect the interference light favorably. Further, if thesubjected eye 100 moves while a tomographic image thereof is captured,artifacts are caused in the captured tomographic image. One of causesfor the subjected eye 100 to move during image capturing is oscillationdue to heartbeats (1 to 2 Hz) of the subject. Therefore, by setting thewavelength swept frequency to 10 Hz or more, the wavelength sweptfrequency becomes faster than the heartbeats of the subject. Thereby,the subjected eye 100 can be suppressed from moving due to theheartbeats of the subject during image capturing, and artifacts in acaptured tomographic image can be suppressed from occurring. In thepresent embodiment, the wavelength swept frequency of the light source12 is 1300 Hz. The wavelength swept frequency of light outputted fromthe light source 12 can be adjusted by a controller 64, of whichconfiguration will be described later.

The measurement optical system is constituted of a beam splitter 24, amirror 28, a 0-point (zero-point) adjustment mechanism 30, a mirror 34,a focal point adjustment mechanism 40 (focal point adjuster 40), amirror 46, and a hot mirror 48. Light outputted from the light source 12is guided to the measurement unit 10 through the beam splitter 18. Thelight guided to the measurement unit 10 enters the subjected eye 100through the beam splitter 24, the mirror 28, the 0-point adjustmentmechanism 30, the mirror 34, the focal adjustment mechanism 40, themirror 46, and the hot mirror 48. Reflected light from the subjected eye100 is guided to the light receiving element 26 through the hot mirror48, the mirror 46, the focal point adjustment mechanism 40, the mirror34, the 0-point adjustment mechanism 30, the mirror 28, and the beamsplitter 24. The 0-point adjustment mechanism 30 and the focal pointadjustment mechanism 40 will be described in detail later.

The reference optical system is constituted of the beam splitter 24 anda reference mirror 22. A part of the light guided to the measurementunit 10 through the beam splitter 18 is reflected by the beam splitter24, is directed to the reference mirror 22, and then is reflected by thereference mirror 22. The light reflected by the reference mirror 22 isguided to the light receiving element 26 through the beam splitter 24.The reference mirror 22, the beam splitter 24, and the light receivingelement 26 are disposed in an interferometer 20, and their positions arefixed. Therefore, in the optical coherence tomographic device of thepresent embodiment, a reference optical path length is constant and doesnot change.

The light receiving element 26 detects the interference light in whichthe light guided by the reference optical system is combined with thelight guided by the measurement optical system. For example, aphotodiode can be used as the light receiving element 26.

The observation optical system 50 irradiates the subjected eye 100 withobservation light through the hot mirror 48 and captures reflected lightthat is reflected from the subjected eye 100 (that is, reflected lightof the observation light). Here, the hot mirror 48 reflects the lightfrom the light source 12 of the interference optical system 14 andtransmits light from a light source of the observation optical system50. As a result, in the optical coherence tomographic device of thepresent embodiment, it is possible to perform the measurement by theinterference optical system 14 and the observation of the anterior partof the eye by the observation optical system 50 at the same time. Anobservation optical system that has been used in a well-known opticalcoherence tomographic device can be used as the observation opticalsystem 50. For this reason, detailed configuration thereof is notexplained herein.

The 0-point adjustment mechanism 30 and the focal point adjustmentmechanism 40 provided in the measurement optical system will beexplained below. The 0-point adjustment mechanism 30 is provided with acorner cube 32, and a second driver 56 (shown in FIG. 2) configured tomove the corner cube 32 back and forth with respect to the mirrors 28and 34. When the second driver 56 moves the corner cube 32 in adirection of an arrow A in FIG. 1, an optical path length from the lightsource 12 to the subjected eye 100 (that is, an object optical pathlength of the measurement optical system) changes. As shown in FIG. 3,when there is an optical path difference ΔZ between the object opticalpath length from the light source 12 to a detection surface (a cornealsurface in FIG. 3) of the subjected eye 100 (more specifically, thelight source 12 to the detection surface plus the detection surface tothe light receiving element 26) and the reference optical path lengthfrom the light source 12 to the reference mirror 22 (more specifically,the light source 12 to the reference mirror 22 plus the reference mirror22 to the light receiving element 26), the larger the optical pathdifference ΔZ becomes, the lower intensity of the interference lightbetween reflected light that is reflected from the detection surface andthe reference light becomes. Conversely, the smaller the optical pathdifference ΔZ becomes, the higher the intensity of the interferencelight becomes. Therefore, in the present embodiment, by changing theobject optical path length with the 0-point adjustment mechanism 30, itis possible to change a position at which the reference optical pathlength and the object optical path length match (that is, the 0-point)from the surface of a cornea 102 to a surface of the retina 106.

The focal point adjustment mechanism 40 is provided with a convex lens42 disposed on a light source 12 side, a convex lens 44 disposed on asubjected eye 100 side, and a third driver 58 (shown in FIG. 2)configured to move the convex lens 44 back and forth with respect to theconvex lens 42 in an optical axis direction. The convex lens 42 and theconvex lens 44 are disposed on the optical axis and change a position ofa focal point of incident parallel light. Thus, when the third driver 58drives the convex lens 44 in a direction of an arrow B in FIG. 1, aposition of the focal point of incident light to the subjected eye 100changes in the depth direction of the subjected eye 100.

By adjusting an interval between the convex lens 42 and the convex lens44 in the focal point adjustment mechanism 40, a diameter D of light atan incident position to the subjected eye 100 (that is, at a position ona front surface of the cornea 102) (hereinbelow, may simply be referredto as “diameter D of light”) can be changed. Specifically, when theinterval between the convex lens 42 and the convex lens 44 is adjustedsuch that light from the convex lens 44 becomes parallel light, theparallel light enters the subjected eye 100 as shown in FIG. 4A.Meanwhile, when the convex lens 44 is moved away from the convex lens 42in the state of FIG. 4A, the light from the convex lens 44 becomesconvergent light and the convergent light enters the subjected eye 100as shown in FIG. 4B. In this case, the diameter D of the convergentlight entering the subjected eye 100 is smaller than the diameter D ofthe parallel light entering the subjected eye 100 (that is, the diameterD in FIG. 4A). Further, when the convex lens 44 is moved closer to theconvex lens 42 in the state of FIG. 4A, the light from the convex lens44 becomes diverging light and the diverging light enters the subjectedeye 100 as shown in FIG. 4C. In this case, the diameter D of thediverging light entering the subjected eye 100 is larger than thediameter D of the parallel light entering the subjected eye 100 (thatis, the diameter D in FIG. 4A). As above, by changing the diameter D oflight, a diameter of light entering the subjected eye 100 can bechanged.

There may be a case where the crystalline lens 104 of the subjected eye100 has an opacified portion 105 due to cataract, for example. FIGS. 5A,5B schematically show light entering the subjected eye 100 in the casewhere the crystalline lens 104 of the subjected eye 100 has theopacified portion 105. For simpler explanation, FIGS. 5A, 5B show thelight entering the subjected eye 100 as parallel light, regardless ofmagnitude of the diameter D of light. As shown in FIG. 5A, when thediameter D of light is small, the light outputted from the light source12 attenuates at the opacified portion 105, by which an amount of lightreaching the retina 106 becomes small. On the other hand, when thediameter D of light is large as shown in FIG. 5B, a part of the lightentering the subjected eye 100 attenuates at the opacified portion 105,while rest of the light entering the subjected eye 100 reaches theretina 106 without passing the opacified portion 105. Due to this, evenwith the subjected eye 100 having the crystalline lens 104 with theopacified portion 105, a signal strength of reflected light from theretina 106 can be increased by making the diameter D of light enteringthe subjected eye 100 large. The diameter D of light may be 3 mm or moreand equal to a pupil diameter of the subjected eye 100 or less. With thediameter D of light of 3 mm or more, the light can easily reach theretina 106. Further, with the diameter D of light equal to the pupildiameter of the subjected eye 100 or less, the light can enter inside ofthe eye efficiently. When a measurement is performed, the pupil diameterof the subjected eye 100 may be increased by a mydriatic agent and thelike. By increasing the pupil diameter of the subjected eye 100, thediameter D of light can be increased.

The K-clock generator 60 optically generates a sample clock (K-clock)signal from the light split through the beam splitter 18 to sample aninterference signal at equal-interval frequencies (at uniform frequencyintervals with respect to light frequency). The generated K-clock signalis outputted to the controller 64. Due to this, distortion in theinterference signal can be suppressed, and deterioration in resolutioncan be prevented.

Further, the optical coherence tomographic device of the presentembodiment is provided with a position adjuster 16 (shown in FIG. 2)configured to adjust a position of the measurement unit 10(specifically, the optical system in the measurement unit 10 excludingthe interferometer 20) with respect to the subjected eye 100, and afirst driver 54 (shown in FIG. 2) configured to drive the positionadjuster 16. A position adjustment process by the position adjuster 16will be described later.

Next, a configuration of a control system of the optical coherencetomographic device of the present embodiment will be described. As shownin FIG. 2, the optical coherence tomographic device is controlled by thecontroller 64. The controller 64 is constituted of a microcomputer (amicroprocessor) including a CPU, a ROM, a RAM, and the like. The lightsource 12, the first to third drives 54 to 58, a monitor 62, and theobservation optical system 50 are connected to the controller 64. Thecontroller 64 controls on/off of the light source 12 and also controlsthe wavelength swept frequency of light outputted from the light source12. Therefore, the controller 64 can change the wavelength sweptfrequency of light outputted from the light source 12. Further, thecontroller 64 drives respective mechanisms 16, 30, and 40 by controllingthe first to third drives 54 to 58, and displays on the monitor 62 animage of the anterior part of the eye captured in the observationoptical system 50 by controlling the observation optical system 50.

As described above, the controller 64 is configured to adjust thewavelength swept frequency of the light outputted from the light source12, and is configured to adjust the diameter D of light entering thesubjected eye 100 by driving the third driver 58 to drive the focaladjustment mechanism 40. Thus, the controller 64 can adjust a wavelengthswept frequency S of light outputted from the light source 12 and thediameter D of light entering the subjected eye 100, such that D/S×λ>1.61is satisfied. In this conditional expression, S indicates a wavelengthswept frequency and λ indicates a wavelength of light. When the diameterD of light is increased to satisfy D/S×λ>1.61, the amount of lightreaching the retina 106 can be increased. Further, when the wavelengthswept frequency S is decreased to satisfy D/S×λ>1.61, the detectionsensitivity for light received by the light receiving element 26 isimproved. Conventionally, the subjected eye 100 had been measured withhigh wavelength swept frequency S to enhance processing capacity.However, as a result of researches by the inventor of the techniquedisclosed herein in a perspective of obtaining favorable tomographicimages even with the subjected eye 100 having the crystalline lens 104with the opacified portion 105, it has been revealed that a relationshipbetween the diameter D of light, the wave swept frequency S, and thewavelength λ affects the measurement. Then, as a result of furtherresearch by the inventor on the relationship between the diameter D oflight, the wave swept frequency S, and the wavelength λ, it has beenalso revealed that when light satisfying D/S×λ>1.61 is used, a favorablemeasurement result can be obtained even with the subjected eye 100having the crystalline lens 104 with the opacified portion 105.

The controller 64 is configured to change (adjust) the wavelength sweptfrequency S of the light outputted from the light source 12 in thepresent embodiment, however, no limitation is placed thereto. Thecontroller 64 may be configured to adjust the wavelength λ of the lightoutputted from the light source 12, or may be configured to adjust boththe wavelength swept frequency S and the wavelength λ.

Further, the light receiving element 26 and the K-clock generator 60 areconnected to the controller 64. An interference signal according to theintensity of interference light detected in the light receiving element26 is inputted to the controller 64. Further, the K-clock signalgenerated in the K-clock generator 60 is inputted to the controller 64.The controller 64 samples the interference signal from the lightreceiving element 26 based on the K-clock signal. That is, thecontroller 64 functions as an example of “sampling circuit” in thepresent embodiment. Then, the controller 64 performs Fourier transformon the sampled interference signal to specify the positions ofrespective parts of the subjected eye 100 (front and rear surfaces ofthe cornea 102, front and rear surfaces of the crystalline lens 104,front surface of the retina 106) and to calculate an axial length of thesubjected eye 100.

Next, a procedure for capturing an optical tomographic image of thesubjected eye 100 having the crystalline lens 104 with the opacifiedportion 105 by using the optical coherence tomographic device of theembodiment will be described. Firstly, an examiner operates an operationmember, which is not shown, such as a joystick to position themeasurement unit 10 with respect to the subjected eye 100. That is, thecontroller 64 drives the position adjuster 16 by the first driver 54 inaccordance with the examiner's operation to the operation member. Due tothis, a position of the measurement unit 10 in xy-directions(vertical-horizontal directions) and a position thereof in a z-direction(a direction in which the measurement unit 10 moves back and forth) areadjusted with respect to the subjected eye 100. Further, the controller64 drives the second driver 56 to adjust the 0-point adjustmentmechanism 30. Due to this, a position of the 0-point at which the objectoptical path length matches the reference optical path length comes tobe positioned at a predetermined position in the subjected eye 100 (forexample, at the front surface of the cornea 102).

Next, the controller 64 drives the third driver 58 to adjust the focaladjustment mechanism 40. Due to this, the diameter D of light enteringthe subjected eye 100 is adjusted. Further, the controller 64 adjuststhe wavelength swept frequency S of the light outputted from the lightsource 12. More specifically, the controller 64 adjusts the diameter Dof light and the wavelength swept frequency S such that D/S×λ>1.61 issatisfied. The controller 64 may adjust one of the diameter D of lightand the wavelength swept frequency S such that D/S×λ>1.61 is satisfied.

Then, the controller 64 takes in the signal detected by the lightreceiving element 26, while changing the frequency of light from thelight source 12. As has already been explained, when the frequency oflight from the light source 12 changes, the position where themeasurement light interferes with the reference light and aninterference wave is generated changes in the depth direction of thesubjected eye 100. Therefore, the interference signal outputted from thelight receiving element 26 becomes a signal of which strength changeswith time as shown in FIG. 6, and this signal includes signals createdby the interference wave between the reference light and reflectedlights from the respective parts (the front and rear surfaces of thecornea 102, the front and rear surfaces of the crystalline lens 104, thesurface of the retina 106) of the subjected eye 100. The controller 64performs the Fourier transform on the signal inputted from the lightreceiving element 26 to separate, from that signal, interference signalcomponents created by the reflected lights from the respective parts(e.g., the front and rear surfaces of the cornea 102, the front and rearsurfaces of the crystalline lens 104, the surface of the retina 106) ofthe subjected eye 100. Due to this, the controller 64 can specify thepositions of the respective parts of the subjected eye 100.

Specific examples of the disclosure herein have been described indetail, however, these are mere exemplary indications and thus do notlimit the scope of the claims. The art described in the claims includesmodifications and variations of the specific examples presented above.Technical features described in the description and the drawings maytechnically be useful alone or in various combinations, and are notlimited to the combinations as originally claimed.

What is claimed is:
 1. An optical coherence tomographic deviceconfigured to acquire a tomographic image of a subjected eye, theoptical coherence tomographic device comprising: a light source ofwavelength sweeping type; and a measurement optical system configured toirradiate the subjected eye with light outputted from the light source,wherein D/S×λ>1.61 is satisfied, with a diameter of the light outputtedfrom the light source at an incident position to the subjected eye is D,a wavelength swept frequency of the light source is S, and a centerwavelength of the light outputted from the light source is λ.
 2. Theoptical coherence tomographic device according to claim 1, wherein thediameter D is 3 mm or more.
 3. The optical coherence tomographic deviceaccording to claim 1, wherein the wavelength swept frequency S is 10 Hzor more and 5000 Hz or less.
 4. The optical coherence tomographic deviceaccording to claim 1, wherein the center wavelength λ is 700 nm or moreand 1400 nm or less.
 5. The optical coherence tomographic deviceaccording to claim 1, further comprising a controller configured tocontrol the light source, wherein the light source is configured to becapable of adjusting at least one of the wavelength swept frequency Sand the center wavelength λ, and the controller is configured to adjustat least one of the wavelength swept frequency S and the centerwavelength λ of the light outputted from the light source.
 6. Theoptical coherence tomographic device according to claim 1, wherein themeasurement optical system is configured to be capable of adjusting thediameter D of the light outputted from the light source at the incidentposition to the subjected eye, and the measurement optical system isconfigured to adjust the diameter D so that the diameter D becomes equalto or less than a pupil diameter of the subjected eye.
 7. The opticalcoherence tomographic device according to claim 6, wherein themeasurement optical system comprises a focal point adjuster configuredto be capable of adjusting a focal position of the light outputted fromthe light source.
 8. The optical coherence tomographic device accordingto claim 1, further comprising: a light receiving element configured toreceive reflected light from the subjected eye and to output a signalwhich corresponds to an intensity of the reflected light; a samplingcircuit configured to sample the signal outputted from the lightreceiving element; and a sample clock generator configured to generate aclock signal which defines a timing for sampling the signal based on afrequency of the light outputted from the light source.
 9. A lightsource of wavelength sweeping type that is equipped in an opticalcoherence tomographic device configured to acquire a tomographic imageof a subjected eye and is configured to output light with which thesubjected eye is irradiated, wherein with a diameter of light outputtedfrom the light source at an incident position to the subjected eye is D,a wavelength swept frequency is S, and a center wavelength is λ, thelight source is configured to be capable of adjusting at least one ofthe wavelength swept frequency S and the center wavelength λ so thatD/S×λ>1.61 is satisfied.
 10. An optical coherence tomographic deviceconfigured to acquire a tomographic image of a subjected eye, theoptical coherence tomographic device comprising: a light source ofwavelength sweeping type; and a measurement optical system configured toirradiate the subjected eye with light outputted from the light source,wherein a diameter D of the light outputted from the light source at anincident position to the subjected eye is 3 mm or more.